# speed up a code, subindex for 4-d matrix?

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MementoMori on 4 Apr 2023
Answered: Rasmita on 14 Apr 2023
Hi, in my code I need to do this operation,
for a = 1:h
for b = 1:h
ind=sub2ind(size(I), centriX+a - h/2, centriY+ b - h/2);
A(:,:,a,b) = (I(ind));%*gamma1;
end
end
where I is a 2D matrix, "centriX" and "centriY" are 2D matrices that contains the coordinates of some centers that I am interested in.
Is there any way not to use for loops?
I have uploaded the files of "centriX" and "centriY".
Matrix "I" is too heavy, but you can take a random matrix (5200x5200). h=40
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MementoMori on 4 Apr 2023
I have edited the question

Rasmita on 14 Apr 2023
Hi,
It is my understanding that you are trying to rewrite the above code without using loops. You can use the bsxfun function in MATLAB to compute the required indices for all combinations of a and b in one go, without using loops.
In the first step, you can create the indices ‘a’ and ‘b’ relative to the center point. In the second step, you can use bsxfun to add these indices to the corresponding elements of centriX and centriY, resulting in a 4D array of shape (numel(centriX), numel(centriY), h, h) containing the indices for all combinations of a and b. Finally, extract the values from matrix ‘I using linear indexing, then reshape the result to the desired shape, and assign it to A.
Hope this helps!
Regards,
Rasmita